1. Field of the Invention
This invention relates to a heavy wall monolithic iron pipe comprising nodular graphite in the outer portion of its wall and flake or quasi-flake graphite in the inner portion of its wall and a method for making said pipe. The central portion of the wall of the pipe will be a combination of nodular graphite and flake or quasi-flake graphite.
In sulfuric acid plants, sulfuric acid at a concentration of 95% or greater and at temperatures of about 250.degree.-300.degree. F. must be piped throughout the plant for processing. The problems associated with the pumping of such concentrated sulfuric acid include corrosion-erosion of the pipes used for transporting the liquid. Not only must the pipe be resistant to the erosion caused by the rapid flow of concentrated sulfuric acid, but the pipe must also have sufficient strength to prevent their collapse.
It is known that for transporting sulfuric acid at room temperature, steel pipe may be used to transport the concentrated sulfuric acid. However, at the elevated temperatures of 250.degree.-300.degree. F., steel pipe quickly corrode-erode and are not practical.
On the other hand, gray cast iron pipe with heavy walls have been used because such pipe resist corrosion-erosion caused by the action of the concentrated sulfuric acid better than either steel or ductile iron pipe. However, gray cast iron does not have the strength of ductile iron for the same wall thickness. Wall thicknesses, generally used for sulfuric acid piping, range from about 1/2 inch for 6 inch pipe to approximately 1 inch for 24 inch pipe. This is twice the thickness of nodular iron pipe used for water and wastewater service.
On the other hand, ductile iron pipe has greater strength than gray cast iron pipe. When sulfuric acid moves relatively slowly through the pipe at 3-4 feet per second, ductile iron pipe and gray cast iron pipe have about the same erosion resistance. However, when the sulfuric acid moves at a rate of 10 to 12 feet per second, gray iron pipe outperforms ductile iron pipe and its erosion resistance exceeds that of ductile iron pipe.
While the applicant does not wish to be bound by any scientific explanation of why flake or quasi-flake graphite iron is less erosion resistant than nodular graphite iron, it may be suggested that the graphite in flake or quasi-flake structure will form a mat or interfacing structure which will not be attacked by the hot sulfuric acid. While the metal of the pipe may be corroding, the mat remains to slow down the rate of erosion caused by flow of the sulfuric acid. On the other hand, nodular graphite, while it contributes to the strength of the pipe, does not have the interfacing structure to inhibit erosion caused by the flow of the sulfuric acid.
Although more erosion resistant, flake or quasi-flake graphite fails to provide the strength of spheroidal or ductile graphite because the elongated form of the flake or quasi-flake graphite causes planes of weakness in the iron pipe to degrade its strength.